CO and Hydrocarbon Oxidation on Pd/Ceria-Zirconia/Alumina Three-way Catalysts

Three-way catalyst (TWC) light-off (LO) performance is critical in cost effectively meeting automotive emission standards. Improvements in catalyst LO should include lower precious group metal (PGM) loading, use of a lower cost PGM such as Pd, and catalyst modifications and components that minimize inhibition effects of exhaust species and enhance catalyst activity. The use of ceria-zirconia (CeO2-ZrO2) within the TWC provides the key benefit of oxygen storage, compensating for deviations from stoichiometric operating conditions by supplying oxygen for CO and hydrocarbon oxidation under rich transient conditions. Pd/Al2O3 and Pd/Ceria-Zirconia monolith catalysts were synthesized and catalyst performance experiments were conducted to study the role of Pd and support compositions and loadings on inhibition during oxidation of CO, acetylene (C2H2), and propylene (C3H6) under near-stoichiometric conditions (λ = 1.01). Transient and steady-state bench scale reactor studies with simulated exhaust gas mixtures were conducted to extract oxidation kinetics, compare LO behavior, and develop a predictive model for understanding and optimizing catalyst performance.

CO, C2H2, and C3H6 were observed to be self-inhibiting. For the same concentrations of carbon (ppm C1 basis) in the feed mixture, C2H2 oxidation occurs at higher temperatures than C3H6 oxidation. Mutual inhibition effects were also examined for binary and ternary mixtures of CO, C2H2, and C3H6; C2H2 was observed to strongly inhibit CO and C3H6 oxidation in mixtures. For singular and mixture oxidation experiments, LO temperatures were lower using the Pd/Ceria-Zirconia catalyst compared to using Pd/Al2O3, demonstrating the promotional effects of using
ceria. Steady-state kinetics measurements during individual species oxidation were conducted to
find reaction orders and activation energies of oxidation. Ongoing work involves the use of a
mechanistic-based kinetic model incorporated into a low-dimensional monolith reactor model.
The reactor model comprises coupled, transverse-averaged species and energy balances and
aforementioned kinetics. The reactor model is used to verify and predict experimental LO
behavior and trends during individual and co-oxidation using catalysts and feed mixtures of
different formulations.